[swift-evolution] Guaranteed closure execution

[Chris Lattner]

On Jan 29, 2016, at 12:23 AM, Jacob Bandes-Storch via swift-evolution <swift-evolution at swift.org> wrote:
> I've wanted something like this as well. I think it would be harder than it seems, because "x = 1" might need to perform initialization, or assignment, depending how it's used.
> 
> It could make sense to have something like "@noescape(executed_exactly_once)" but this might be so limited it's not worth it. And I'm not sure how it should interact with throws.

I think that something like this is implementable, and making it a modifier to @noescape is sensible.

The semantics we could support is that the function is guaranteed to call the closure exactly once on any path that could lead to a return or throw.

This approach allows you to pass the closure down the stack, and composes with error handling.  It is obviously limited what you can do with the closure, but that is necessary to validate correctness.

-Chris



> 
> Jacob
> 
> On Thu, Jan 28, 2016 at 11:38 PM, Gwendal Roué <swift-evolution at swift.org <mailto:swift-evolution at swift.org>> wrote:
> Hello,
> 
> I’d like to discuss the opportunity to let functions declare that a closure argument is guaranteed to have been executed when the function has returned.
> 
> For example:
> 
> 	func f(@noescape(executed) closure: () -> ()) {
> 	    closure()
> 	}
> 
> The expected advantage is that the compiler would know that a variable set inside the closure is guaranteed to be initialized, and that it can be used after the execution of the function, as below:
> 
> 	let x: Int  // Not initialized
> 	f { x = 1 }
> 	print(x)    // Guaranteed to be initialized
> 
> Today developers have to write pessimistic code like below:
> 
> 	var x: Int = 0 // `var` declaration, with some irrelevant value
> 	f { x = 1 }
> 	print(x)
> 
> As for a real world usage, I’d like to access a database in a safe (queued) way, and fetch values out of it:
> 
> 	let items: [Item]
> 	let users: [User]
> 	dbQueue.inDatabase { db in
> 	    items = Item.all().fetchAll(db)
> 	    users = Item.all().fetchAll(db)
> 	}
> 
> Gwendal Roué
> 
> 
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